At present, it is believed that many intracranial infections are caused by viruses. In addition to various acute encephalitis with clear virus types, there are also slow virus infections or slow virus diseases, whose main lesions are chronic disease processes of demyelination and (or) degeneration. Pathological changes mainly include direct damage from the virus and pathological reactions of tissues, the latter of which is the result of the body's immune response to viral antigens. Due to different types of viruses (such as strains), various pathological reactions are caused. The nature and degree of the lesions are directly related to the pathogens of infection and the body's response. Sporadic (viral) encephalitis is not an independent disease. Based on clinical practice, the current clinical classification is mostly based on the onset form (acute or slow) and the important characteristics of pathological changes, which are basically divided into two categories: ① Acute viral encephalitis: including epidemic encephalitis B, herpes simplex virus encephalitis, etc.; ② Slow virus encephalitis: subacute sclerosing panencephalitis, progressive multifocal leukoencephalopathy, etc. are known.